Polarization-preserving image rotator.

A K-mirror rotates the wavefront of an incident optical field. However, the rotation always introduces polarization changes in the transmitted field. This is a serious concern for applications ranging from astronomical image derotation to orbital angular momentum characterization in photonic quantum technology. Recent efforts have provided ways to minimize polarization changes, but these require either a very small base angle that limits the field of view, or mirrors with a customized refractive index. Nonetheless, image rotation without any associated polarization change has remained an open problem. In this work, we show that placing half-wave plates before and after a K-mirror and rotating them synchronously at half the K-mirror rotation angle makes the polarization change in the transmitted field completely independent of the rotation angle. Using a K-mirror with a base angle of 30∘, which gives the largest field of view among practical designs, we experimentally demonstrate this, within the mean polarization error of ∼1% caused mostly by the retardance error of commercially available half-wave plates. Our scheme works at any base angle and mirror refractive index. More importantly, it works not only for K-mirrors but also for any type of image rotator. Thus, it can have important implications for applications requiring precise wavefront rotation without polarization change.